In recent years, with electric power shortage becoming serious, there have been challenges that are global rapid adoption of natural energy such as the adoption of wind power generation and photovoltaic power generation and the stabilization of power systems (for example, maintaining of frequency and voltage). One technique for addressing the challenges has been attracting attention and this technique is to install high-capacity secondary batteries to achieve, for example, smoothing of output variations, storage of surplus power, and load leveling.
One of such high-capacity secondary batteries is a redox flow battery (hereafter, sometimes referred to as an RF battery). The RF battery has the following features, for example: (1) a high capacity in the megawatt class (MW class) is easily achieved, (2) the long life, and (3) the state of charge of the battery can be accurately monitored. Thus, the RF battery is expected to be optimum as a secondary battery used for the stabilization of power systems.
The RF battery includes a battery cell having a positive electrode, a negative electrode, and a membrane interposed between the electrodes and is charged and discharged while a positive electrode electrolyte and a negative electrode electrolyte are supplied to the battery cell. The electrolytes for the electrodes are typically solutions containing, as active materials, metal ions that undergo changes in valence by oxidation-reduction. Typically, there are an Fe—Cr-based RF battery employing iron (Fe) ions as the positive electrode active material and chromium (Cr) ions as the negative electrode active material, and a V-based RF battery employing vanadium (V) ions as the active material of the two electrodes (Paragraph 0003 in Description of Patent Literature 1).
Patent Literature 1 discloses, as an RF battery that can provide a higher electromotive force than the existing V-based RF batteries, a Mn—Ti-based RF battery employing manganese (Mn) ions as the positive electrode active material and titanium (Ti) ions or the like as the negative electrode active material. Patent Literature 1 also discloses that titanium ions are additionally contained in the positive electrode electrolyte, so that generation of precipitate such as manganese oxide (MnO2) can be suppressed and the reaction of Mn2+/Mn3+ can be performed with stability.